The invention relates to a method for manufacturing a substrate for the placement of electrical and/or electronic components thereon. Particularly the method according to the present invention concerns the manufacture of substrates with good thermal and mechanical properties that are especially suited for use in hybrid modules for power electronics.
In practice, a substrate of the above-mentioned type has the same function as a printed circuit board. Common printed circuit boards for placement of electronic components are usually manufactured of an epoxy material on which is provided a predetermined electrical conducting pattern by means of electroplating. For economical reasons and to reduce the requirement for space, the conducting pattern is usually provided on both sides of the board and the electrical conducting connection between the conducting pattern on each side is provided by through-plated holes in the epoxy substrate. Such circuit boards are however best suited for electronic equipment where low voltages and small currents are used and where the equipment is not subjected to mechanical and thermal loads to any degree.
Substrates for use in electrical engineering or electronics where high voltage and large currents are used and where the equipment further may be subjected to large mechanical and thermal loads are often metallic substrates for instance in form of a sheet of steel and aluminium in order to achieve good thermal conduction and sufficient mechanical strength. Electrical isolation between the conducting pattern and the metal substrate is provided by coating it with an isolating layer, for instance in the form of a enamel coating, alumina coating or a combination of oxides, nitrides of carbonitrides. The isolation layer may also be generated by applying an epoxy resin layer to the metal substrate. The electrical isolating layer may be applied on both sides of the metal substrate or only on one side, the substrate it will be used in modules of power electronics, the metal substrate in that case usually is provided with or has cooling ribs on the other side in order to transport heat away. The electrical isolation layers are provided with an electrical conduction pattern or current leads by means of different processes, for instance by chemical methods or by vapour deposition. Examples of circuit boards of this type are for instance disclosed in DE-PS 3447520 and DE-OS 2556826.
From GB-PS 2110475A there is further known the use of a substrate in the form of an alloy of Fe, Cr, Al and Yt where a ceramic surface layer is formed by the substrate being heated in an oxidizing atmosphere.
The known circuit boards or substrates as well as the methods used in their manufacture are however burdened with a number of disadvantages. Different processes are used in application of the isolating and the conducting layer and those processes are often of such a nature that they in a negative way influence the properties of already applied coatings. It has proved to be difficult to achieve dielectric layers with satisfactory dielectric, thermal and mechanical properties and when these properties then are influenced in a negative way by the process of applying a conducting pattern or by soldering of components thereto, the finished product hardly meets the requirements that are demanded of it e.g. hybrid modules in power electronics. It is for instance difficult to achieve a satisfactory bond between the different layers or the isolating layer and the metal substrate which forms the core or base the dielectric strength of the electrical isolating layer is not as good as desired. One solution might be to increase the thickness of the electrical isolating layer, but this is an expensive solution and in the case of ceramic coatings it does in any case not lead to any improvement, as the coatings usually are brittle and have a relatively low tensile and shear strength, although they are hard materials and in theory have a high dielectric strength. The last property however is reduced due to the formation of pores and impurities caused by migration. It has also turned out that ceramic layers applied by means of, for instance, plasma spraying do not attain better dielectrical properties if the thickness is increased beyond 0.3 mm. In addition the use of different processes for applying the separate layers to the metal substrate insert and impairs manufacturing economy.
The object of the present invention therefore is to avoid the above-mentioned disadvantages, by providing a method for the manufacture of substrates having high mechanical and dielectrical strength and that are able to sustain great thermal loads and in which the same process may be used for coating a base, preferably of a metallic material, with both the electrical isolating and conducting layers.